Wind turbines include a rotor that supports a number of blades extending radially therefrom for capturing the kinetic energy of the wind and causing a rotational motion of a driving train coupled to an electric generator for producing electrical power.
The amount of energy produced by wind turbines is dependant on the rotor blade sweeping surface that receives the action from the wind and consequently increasing the length of the blades leads normally to an increase of the power output of the wind turbine.
The energy produced by a wind turbine depends, among other factors, on the diameter of the rotor and on the characteristics of the wind at the site where the wind turbine is installed.
To optimize the energy produced at the wind turbine site, the usual procedure in selecting the most appropriate wind turbine is the following:                Identifying the wind characteristics at the site (maximum speed, average speed, turbulence . . . ).        Determining the site class (I, II or III) according to international standards (IEC-61400-1).        Selecting the suitable wind turbine model for the site. The wind turbine manufacturers usually offer wind turbines having specific blades for each class. In low wind sites they have larger blades than in high wind sites, so that the loads they induce on the wind turbine are similar.        
This process leads to a very wide classification of sites (only three levels), so that if a site is midway between the limits of two classes an oversized wind turbine can be selected. In this case, the choice of the optimum diameter of the rotor for the intermediate subclass, would entail an optimization of the annual energy production (AEP).
The adaptation of a wind turbine to the site constrains including the selection of a blade length adapted to the features of the site it is discussed in US 2009/0169390. It is proposed the provision a kit of parts comprising several modules for assembling a modular rotor blade, wherein the several modules comprise at least one root-type module and at least one tip-type module and at least one further module of the root-type or the tip-type, wherein the at least one further module has a different shape compared to the other module of the same type. Finally a method for adapting a rotor of a wind turbine to a site constraint is provided.
Therefore the teaching of US 2009/0169390 is limited to the provision of a number of modules of wind turbine blades, manufactured according to standard procedures, for selecting a suitable set for assembling a blade adapted to a site constraint.
The problem of this approach is that it does not provide blades with the optimum length for each particular site.
This invention is addressed to the solution of this problem.